feat: add mixed numbers audios for the A1 Spanish curriculum

[estefaniacn]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

This Pull Request adds two audio files with mixed numbers for the blocks of two modules:

• The "First Numbers" module of the "Spanish Fundamentals" Chapter.
• The "Numbers 10 to 29" module of the "Basic Personal Details" Chapter.